Ices, Oceans, and Fire: Satellites of the Outer Solar System (2007) 6006.pdf

COMPARATIVE ANALYSIS OF LONGITUDINAL DUNES ON SATURN’S MOON TITAN AND THE NAMIB DESERT, NAMIBIA. C. Spencer1, J. Radebaugh1, R. Lorenz2, S. Wall3, J. Lunine4, and the Cassini Radar Team, 1Brigham Young University, Department of Geological Sciences, Provo, UT 84602 [email protected], 2Johns Hopkins University Applied Physics Lab, Laurel, MD, 3Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109, 4Lunar and Planetary Laboratory, Univ. of Arizona, Tucson, AZ 85721.

Sand Seas on Titan: Beginning in 2005, the Cassini Titan Radar Mapper has discovered sand seas made of thousands of longitudinal dunes in the equatorial regions of Saturn’s Moon Titan [1,2,3]. The dune forms observed by Cassini are similar in morphology and scale to longitudinal dune fields found on Earth [2,4,5,6]. These dunes are concentrated in the low latitudes and appear to cover as much as 40% of these terrains and 20% of Titan’s total surface [4,5,6]. The longitudinal dunes of Titan have heights of roughly 100 meters, widths of 1-2 kilometers and lengths from <5 to nearly 150 kilometers [2,3,4,5]. In order to better understand the formation of these landforms and the implications for wind strength and direction on Titan, we analyze similar features found in the Namib Desert on Earth. Namib Desert Geography and Climate: The Fig. 1. Dune/topography interaction on Titan (left) and Earth (Namib) (right). Namib Desert is located on the southwest coast of Namibia in southwest Africa. For the most part the near-by highlands [5] (Fig. 1). Similar features are seen desert is composed of a massive sand sea covering over in the Namib Desert. Where sand supply is low and/or 34,000 km2 [7]. The sand sea is made up of mostly topographic obstacles are present, longitudinal dunes longitudinal or linear dunes with minor transverse, dominate, with only minor transverse dunes [2,5]. barchan, and star dunes [7]. Precipitation in the Namib Dynamics of Longitudinal Dunes: It has been Desert ranges from 15mm/yr to the south, 14mm/yr to widely accepted that two principal wind directions the north, and 23mm/yr in the interior. The Namib has a perpendicular to one another are responsible for the general wind direction from south-southwest to formation of the longitudinal dunes; however, much southwest along the coast and swings around inland controversy has surrounded the formation of these southwest to west southwest for the greater part of the dunes [7,10,11]. Unidirectional wind hypotheses were year (January to March and October to December); proposed in earlier years, but have since been however, during April to September periodic winds discredited [7,11]. blow from the north-northeast to east-southeast [8]. The term longitudinal or linear dune was sub- Longitudinal Dunes of the Namib Desert: divided by [11] into lee dunes (small dunes that form The longitudinal (linear) dunes of the Namib behind an obstacle), vegetated linear dunes (covered by Desert range in size between 100-150 meters high [9] vegetation with a rounded profile), and seif dunes and between 1-3 kilometers apart (measured from (forms from bidirectional winds). Because in the Namib Landsat photographs and [7]). These dunes are Desert and on Titan we have neither dunes covered extremely large in comparison to other longitudinal with vegetation nor small dunes forming behind dunes around the world, which reach only 15-35 meters obstacles, the most compelling hypothesis for the high [7]. In comparison to the dunes on Titan the dunes formation of these longitudinal dunes is the in Namibia come closest in comparison considering bidirectional wind hypothesis. From the formation of their size and morphology. Where sand supply is less longitudinal seif dunes, it is affected by wind blowing and/or topography creates an obstacle, barchan, obliquely from both sides of its slopes, meeting the transverse, and star dunes are also present with dune at an acute angle and separated over the crestline. parabolic (blowout) dunes near the shoreline south of Each wind is diverted at the lee slope to blow parallel to Walvis Bay. the crest in a down-dune direction [11]. Evidence for Dune Interaction with Topography: Within the this hypothesis comes from many different longitudinal sand seas on Titan we see how the dunes interact with dune fields around the Earth, including Australia [13], the surrounding topography; diverging and converging Namibia [7], and the Sahara [14]. around highlands and narrowing together between two Ices, Oceans, and Fire: Satellites of the Outer Solar System (2007) 6006.pdf

Comparative Analysis of Dune Forms on Earth Because the dunes forms seen in the Namib Desert and Titan: Similarities in the presumably ongoing are so similar to those on Titan, a detailed macro- and behavior of dunes around topography on Titan and in microanalysis of these dunes on Earth will enable a the Namib can be seen in Fig. 1. We also seek to more concise model for the genesis, evolution, and understand the evolution of Titan’s dunes to their morphology of the dunes found on Titan. current state. Some have claimed [11,12] some References: [1] Elachi e.a. 2005. LPSC XXXVI, longitudinal dunes are the products of the evolution of Abst #2294. [2] Lorenz R. e.a. 2006. Science, 312:724- barchan and transverse dunes seen in many parts of the 27. [3] Boubin, e.a. 2005. Am. Astr. Society, DPS world’s deserts [15,16,17] including Namibia [8] and meeting 37:723. [4] Radebaugh, J. e.a., 2006. Am. Astr. the Rub al-Khali (Fig. 2). Due to the lack of barchan Society, DPS mtg 38, Abst. 52.07. [5] Radebaugh, J. and transverse dunes on Titan the analog might fall e.a., submitted. Icarus, in revision. [6] Lorenz R. e.a. short. However, because waves and/or fluvial processes 2007. European Geophys. Union Mtng. Abst., Vol. eventually destroy the longitudinal dunes on Earth, we 9:04604. [7] McKee, E.D. 1982. USGS Spec. Pap. 188. are unable to see these dunes in an advanced, evolved [8] Lancaster, N. 1980. Z. f. Geomorph., 24:2:160-67. state. Conversely, if the dunes on Titan originally began .[9] Lancaster, N. 1981. Sedimentology, 1:1-8. [10] as barchonoid and transverse dunes only to evolve into Mainguet, M. 1984. in El-Baz, F. (ed.), Deserts and arid longitudinal dunes, and with no wave and minor fluvial lands. Martinus Nijhoff Publishers, pp. 31-58. [11] processes to speak of in the region of the dunes, these Tsoar, H., 1989. Prog. in Phys. Geog., 13:4:507-28. dunes will have continued to migrate around the [12] Bagnold, R. 1942. NY, W. Morrow. [13] Mabbutt, equatorial regions of the planet developing into what J.A. Sullivan, M.E., 1968. Austrl. Geog., 10:483-87. can be considered highly evolved and mature dune [14] Price, A.W., 1950. Geogr. Rev., 40:462-65. [15] forms that we see now. This is not the only way in Kerr, R. e.a., 1952. AAPG 36:1541-1573. [16] Tsoar, which linear dunes develop [7,10], but due to the H., 1974. Z. f. Geomorph., 12:200-20. [17] Tsoar, H., apparent lack of other dune types on Titan, this method 1984. Z. f. Geomorph., 28:99-103. may be most applicable.

Fig. 3.: Satellite Image of the Rub al Khali, Saudi Arabia. Note the evolution of longitudinal dunes from barchan and transverse dunes. Wind directions obtained from: Dabbagh e.a. 1997. JPL Unpub: http://southport.jpl.nasa.gov/reports/finrpt/Dabbagh/dabbagh.htm.